Relaxation and transfer of strained layers

ABSTRACT

The invention relates to a process for fabricating a heterostructure. This process is noteworthy in that it comprises the following steps: a) a strained crystalline thin film is deposited on, or transferred onto, an intermediate substrate; b) a strain relaxation layer, made of crystalline material capable of being plastically deformed by a heat treatment at a relaxation temperature, at which the material constituting the thin film deforms by elastic deformation is deposited on the thin film; c) the thin film and the relaxation layer are transferred onto a substrate; and d) a thermal budget is applied at at least the relaxation temperature, so as to cause the plastic deformation of the relaxation layer and the at least partial relaxation of the thin film by elastic deformation, and thus to obtain the final heterostructure.

CROSS-REFERENCE TO RELATED APPLICATION

Pursuant to the provisions of 35 U.S.C. 119 this application claimspriority to French Patent Application Serial No. FR0853150 filed May 15,2008, the entire contents of which is hereby incorporated herein by thisreference.

TECHNICAL FIELD

The present invention relates to a heterostructure intended to be usedin applications in the fields of optics, optoelectronics, electronics orphotovoltaic components, which heterostructure comprises a multilayerstack in which the upper layer is in the relaxed or partially relaxedstate.

BACKGROUND

Materials that are not available in bulk form are often fabricated byheteroepitaxy on substrates whose lattice parameter is not perfectlymatched. This results in the formation of tensilely or compressivelystrained materials.

Moreover, when the thickness of the epitaxially grown material exceeds athreshold value, the strains due to the difference in lattice parameterfrom that of the substrate are such that the epitaxially grown materialrelaxes, forming dislocations in the crystal lattice during the epitaxy.These dislocations result in atom stacking faults in the volume of thematerial.

In the case of III/N materials, the dislocations generated by therelaxation are mainly oriented along an axis perpendicular to the planeof the surface of the material. These dislocations are called“through-dislocations,” as they pass through the thickness of the layerof the material. They have a deleterious effect on the performance andlifetime of the components formed from these materials.

Finally, materials obtained by heteroepitaxy and rich in dislocationsconstitute poor seed substrates when they are used for a subsequentepitaxy, since the dislocations that they contain are transmitted to thesubsequent epilayer.

Already known, from document US 2004/0192067, is a process forfabricating a heterostructure that includes a relaxed or partiallyrelaxed useful layer on a substrate.

That process consists in transferring, onto a support, a layer ofamorphous material, such as SiO_(x)N_(y) or SiO₂, which may possiblycontain dopant elements, such as boron or phosphorus, so as to modifythe glass transition temperature of this material in order to make itviscous at the desired temperature. This layer of amorphous material issubjacent to a strained layer. Applying a heat treatment above the glasstransition temperature, enabling the material to pass into the viscousstate, causes partial or complete relaxation of the previously strainedlayer.

However, amorphous materials are electrical insulators. Consequently,the properties of the heterostructure obtained are not always optimizedfor all desired applications.

Moreover, this amorphous layer may contaminate the other layers of theheterostructure or the electronic devices fabricated thereon, by thediffusion of the dopant elements during subsequent heat treatments.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent fromthe following description, with reference to the appended drawings whichshow, by way of indication but implying no limitation, one possibleembodiment.

In these drawings:

FIGS. 1A to 1H are diagrams showing the various successive steps of onepossible embodiment of the process for fabricating a heterostructureaccording to the invention;

FIGS. 2A to 2C are diagrams showing the various successive steps of afirst method of using the heterostructure according to the invention;and

FIGS. 3A to 3C are diagrams showing the various successive steps of asecond method of using the heterostructure according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a heterostructure intended to be usedin applications in the fields of optics, optoelectronics, electronics orphotovoltaic components, which heterostructure comprises a multilayerstack in which the upper layer is in the relaxed or partially relaxedstate.

Such a heterostructure may serve, for example, for the manufacture ofelectronic components such as lasers or LEDs (light-emitting diodes),formed from the upper layer.

Another advantageous application consists in using this heterostructureas an epitaxy substrate in order subsequently to obtain, by epitaxy,relaxed materials of high crystallographic quality, i.e., having aquality sufficient to be able to be used in applications in theaforementioned fields, which materials have in particular a very lowdensity of crystal defects such as dislocations.

The invention also relates to a process for fabricating such aheterostructure.

The invention is particularly applicable in the production of aheterostructure allowing the epitaxy of materials of the III/N type thatare not generally available in bulk form.

The expression “materials of the III/N type” is understood to mean anyalloy of at least one element belonging to column III of the PeriodicTable with nitrogen or, in other words, any nitride of these elementsbelonging to column III.

To give an example of such alloys, mention may be made of aluminiumnitride (AlN), gallium nitride (GaN), indium nitride (InN), and alsoternary or quaternary nitrides such as AlGaN, AlInN, InGaN and AlInGaN.

One object of the invention is to provide a heterostructure comprising amultilayer stack in which the upper layer is in the relaxed or partiallyrelaxed state, while still being of good crystallographic quality, thatis to say, a quality sufficient to allow optimum operation ofelectronic, optoelectronic or photovoltaic components.

Another object of the invention is to provide a heterostructure formingan epitaxy substrate that allows the aforementioned problems of theprior art to be solved and can be used to obtain, by epitaxial growth, amaterial having no or few defects or dislocations.

One particular object of the invention is to provide a heterostructureallowing III/N materials to be obtained in the at least partiallyrelaxed state, it being possible for these materials to be usedsubsequently as epitaxy substrate for obtaining III/N materials in theat least partially relaxed state.

An additional object of the invention is to provide a process forfabricating this type of heterostructure.

For this purpose, the invention relates to a process for fabricating aheterostructure intended to be used in the fields of electronics,optoelectronics, optics or photovoltaic components, whichheterostructure comprises a multilayer stack in which the upper layer isa relaxed or partially relaxed crystalline thin film.

In accordance with the invention, this process comprises the followingsteps:

a) a thin film of crystalline material in the strained state isdeposited epitaxially on, or transferred onto, a substrate called the“intermediate substrate”;

b) a layer of crystalline material, called the “strain relaxationlayer,” is deposited on the thin film, the material constituting therelaxation layer being chosen so as to be plastically deformed by a heattreatment at a temperature called “relaxation temperature,” at which thematerial constituting the thin film deforms by elastic deformation;

c) the assembly made up of the thin film and the relaxation layer istransferred onto a substrate, called the “mechanical support substrate,”so as to form an intermediate heterostructure; and

d) a thermal budget is applied to the intermediate heterostructure inorder to bring it to at least the relaxation temperature, so as to causethe plastic deformation of the relaxation layer of crystalline materialand the at least partial relaxation of the thin film by elasticdeformation, and thus to obtain the final heterostructure.

According to other advantageous and non-limiting features of theinvention, taken individually or in combination:

application of the thermal budget of step d) results in the completerelaxation of the thin film by elastic deformation;

the thickness of the relaxation layer is at least equal to the length ofthe deformation of the thin film when the latter relaxes by elasticdeformation;

prior to step d), an etching step is carried out in the thickness of thethin film in order to form islands of material therein;

during step a), the thin film is transferred onto the intermediatesubstrate, being first epitaxially deposited on a substrate, called an“initial epitaxy substrate,” so as to be in the strained state, and thentransferred onto the intermediate substrate;

the transfer of the thin film onto the intermediate substrate iseffected by the implantation of ionic species through the thin film soas to form a weakened zone within the initial epitaxy substrate, thisweakened zone defining a negative, then by the bonding of the strainedthin film to the intermediate substrate and by a detachment of thenegative from the initial epitaxy substrate by applying an energybudget;

the thin film is deposited on the intermediate substrate and thematerial constituting the thin film has a lattice parameter that differsby at least 0.13% from that of the material of the intermediatesubstrate;

the material constituting the thin film has a lattice parameter thatdiffers by at least 0.13% from that of the material of the initialepitaxy substrate;

step a) is carried out by the thin film being transferred onto ordeposited on the intermediate substrate, the latter being a removablesubstrate;

the thermal budget of step d) is applied by epitaxially depositing anepilayer on the thin film;

the epilayer comprises an active layer;

the thin film is made of a single-crystal material;

the relaxation layer is made of a material chosen from silicon,germanium, indium phosphide and gallium arsenide;

the thin film is made of a material of the III/N type; and

the upper face of the film has the polarity of the III element.

The invention also relates to a heterostructure intended to be used inthe fields of electronics, optoelectronics, optics or photovoltaiccomponents.

In accordance with the invention, it includes a strain relaxation layermade of crystalline material, interposed between a mechanical supportsubstrate and a thin film of crystalline material in the strained state,the material constituting the strain relaxation layer being chosen so asto deform plastically by a heat treatment at a temperature called the“relaxation temperature,” at which the material constituting the thinfilm deforms by elastic deformation and the thickness of the relaxationlayer is at least equal to the length of the deformation of the thinfilm when the latter relaxes by elastic deformation.

According to other advantageous and non-limiting features of theinvention, taken individually or in combination:

it includes a strain relaxation layer made of plastically deformedcrystalline material, interposed between a mechanical support substrateand a thin film of relaxed or partially relaxed crystalline material;

the thin film is in direct contact with the strain relaxation layer;

it includes at least one epilayer made of a material in the relaxedstate on the thin film, the latter being made of single-crystalmaterial;

the thin film is made of a material of the III/N type;

the upper face of the thin film has the polarity of the III element;

the relaxation layer is made of a material chosen from silicon,germanium, indium phosphide and gallium arsenide; and

the thin film is made of InGaN and the strain relaxation layer is madeof germanium.

The process according to the invention includes a first step in which athin film 5 of crystalline material in the strained state is depositedon or transferred onto a substrate 2 called the “intermediatesubstrate,” as shown in FIG. 1D.

There are various possible ways of achieving this result. One of themwill now be described with reference to FIGS. 1A to 1C.

FIG. 1A shows a substrate 1 called an “initial epitaxy substrate,” onthe front face 11 of which a thin crystalline film 5 has been deposited.This film was preferably deposited by heteroepitaxy with pseudomorphicgrowth so that this thin film 5 is in the strained state relative to thesubstrate 1.

As a reminder, it will be recalled that a layer of material is in the“strained” state, in tension or in compression respectively, when itslattice parameter, in the plane of the interface with the material onwhich it rests, is respectively greater than or smaller than its nominallattice parameter, i.e., its lattice parameter in the natural state.

In the present case, the materials constituting the thin film 5 and theinitial epitaxy substrate 1 are preferably chosen so that the differencein their lattice parameters is at least 0.13% for reasons which will beexplained later. This difference in lattice parameter corresponds hereto that existing between the lattice parameter of InGaN containing 0.2%indium and that of relaxed GaN.

Also as a reminder, it will be recalled that the lattice parameters of acrystalline material correspond to the constant distances existingbetween the various atoms of a lattice of this material in threedimensions. They are denoted by a, b and c and are expressed in lengthunits, generally in nanometres.

The parameters a and c correspond to the distances between atoms in thetwo directions of the interface plane between this material and itssubstrate, and the parameter b corresponds to the distance between atomsin a direction different from this plane.

When two materials have a difference in lattice parameter of at least0.13% for example in their interface plane, this means that if the firstmaterial has lattice parameters a and c in this plane of lengths l_(a)and l_(c) respectively, then the second material has lattice parametersa′ and c′ having lengths of l_(a′) and l_(c′) respectively, where:

l _(a′) ≦l _(a)−(0.13×l _(a))/100 or l _(a′) ≧l _(a)+(0.13×l _(a))/100

and conversely:

l _(c)−(0.13×l _(c))/100≦l _(c′) ≦lc+(0.13×l _(c))/100.

Preferably, the thickness of the strained thin film 5 does not exceedthe threshold thickness above which crystal defects appear, such asdislocations, fissures, cracks or even phase separation causingrelaxation during the epitaxial growth or cooling of the multilayerstack at the end of the epitaxy. The fissures or cracks are generallydue to the difference in thermal expansion coefficient of the materialsconstituting the initial epitaxy substrate 1 and the thin film 5, whilethe phase separation is due to the structural instability of certainstrained materials.

It will be noted that the force of the strain induced in the thin film 5is also proportional to the strained volume (diameter x thickness in thecase of a cylindrical film). The stored elastic energy also depends onthe Young's modulus of the material in question.

The material constituting the thin film 5 is a single-crystal orpolycrystalline material. It may be chosen from the materials commonlyused in electronics, for example semiconductor materials. If, asexplained above, the heterostructure fabricated by the process accordingto the invention has in particular the purpose of allowing subsequenthomoepitaxy of III/N materials, the thin film 5 will advantageously bechosen from one of these materials and will be a single-crystalmaterial.

The material constituting the support substrate 1 will be chosenaccording to that of the thin film 5 so as to meet the aforementionedcriteria, in particular in terms of lattice parameters and thermalexpansion coefficient in order to obtain the material 5 in the strainedstate.

To give an example, when the thin film 5 consists of a III/N material,the substrate 1 consists for example of silicon, SiC, sapphire, ZnO, MgOor GaN. It is also conceivable to have an intermediate layer between thesubstrate 1 and the epitaxially grown film 5, such as for example GaN onsapphire. The substrate 1 may be a bulk material or a multilayermaterial, its upper layer at the interface with the film 5 then havingto meet the abovementioned criteria.

Preferably, as shown in FIG. 1B, the strained thin film 5 is transferredonto the intermediate substrate 2 using the technology known by thetrademark SmartCut®. To do this, one or more ionic species are implantedthrough the thin film 5 so as to create a weakened zone 10 in theinitial epitaxy substrate 1. This weakened zone 10 delimits a film 13from the remainder 14 of the substrate 1, called the negative.

Advantageously, the thin film 5 may be covered with an oxide layer (notshown in FIG. 1B), which prevents damage and contamination of theimplanted surface, and also a channelling effect of the implantedspecies. The thin film, whether or not it is covered with an oxidelayer, is then prepared for being bonded to the intermediate substrate2. To do this, the two substrates are brought into intimate contact. Itshould be noted that a bonding layer 71 may be interposed between thetwo, this layer possibly being deposited either on the intermediatesubstrate 2 or on the thin film 5, or on both of these.

As shown in FIG. 1D, an energy budget is then applied to theaforementioned multilayer stack, so as to detach the negative 14 fromthe initial epitaxy substrate 1 in the weakened zone 10. The residuallayer 13 of the substrate 1 then undergoes a polishing, chemical etchingor grinding operation in order to free the surface of the thin film 5.

The nature and the thickness of the intermediate substrate 2 will bechosen so as to have sufficient mechanical strength for transferring thefilm 5. In an alternative embodiment (not shown in the figures), thethin crystalline film 5 in the strained state could be transferred ontoan intermediate substrate 2 of the “removable substrate” type.

The term “removable substrate” is understood to mean any type ofsubstrate that can be easily detached from the thin film 5. Such anintermediate substrate may have the “removable” character because itincludes, for example, a weakened layer or a layer capable of beingselectively etched.

As shown in FIG. 1E, the process then continues with the deposition of alayer 6 of crystalline material, called the “strain relaxation layer” onthe thin film 5. The material constituting the strain relaxation layer 6is chosen so as to be plastically deformed by a heat treatment at whatis called the “relaxation temperature,” at which the materialconstituting the thin film 5 itself deforms elastically, so as toprevent the formation in the film 5 of crystal defects typicallyassociated with a plastic deformation. The elastic deformation of thestrained thin film 5 results in at least partial relaxation.

Although the relaxation of a film can take place by elastic and/orplastic deformation, the object of the invention is to obtain the atleast partial relaxation of the strained film 5 only by elasticdeformation. The term “plastic deformation” is understood to mean anirreversible deformation of the material, which takes place for exampleby the generation of defects and/or fissures. These concepts will bediscussed in detail later.

It should be noted that the difference between the lattice parameter ofthe material constituting the thin film 5 and that constituting thestrain relaxation layer 6 is of little importance since the crystallinequality of the relaxation layer 6 has little impact on the relaxation ofthe material of the strained film 5. The material of the relaxationlayer 6 may be a single-crystal material and may contain many defects,but it is preferably a polycrystalline material. When this material ispolycrystalline, the grain boundaries react as free surfaces andfacilitate the diffusion of atoms and the formation of defects leadingto the plastic deformation of the layer 6.

Preferably, the thickness of the relaxation layer 6 is at least equal tothe length of the deformation of the thin film 5 when the latter, whichwas strained, deforms elastically (i.e., when it relaxes), as will bedescribed later. To give an example, if the lattice parameter of thethin film 5 differs by 0.5% from its nominal lattice parameter and if ithas a diameter of 5 mm, it will elongate by 0.025 mm once in the relaxedstate. The relaxation layer 6 therefore preferably has a thickness of atleast 0.025 mm. Preferably, when the thin film 5 is made of a III/Nmaterial, the relaxation layer 6 is made of a material chosen fromsilicon, germanium, indium phosphide and gallium arsenide.

The strain relaxation layer 6 is then prepared for being bonded to amechanical support substrate 3. The mechanical substrate 3 isadvantageously covered with a bonding layer 72, for example an oxidelayer.

Next, it is advantageous to carry out a heat treatment to enhance thebonding, followed by detachment of the intermediate substrate 2, asshown in FIG. 1G. This detachment step may be carried out by etching theintermediate substrate 2, but in this case the substrate will bedestroyed. It will be preferable to etch the bonding layer 71. As shownin FIG. 1G, it will be noted that when the intermediate substrate 2 hasbeen detached, the strained thin film 5 has, on one side, a free upperface 51 and, on the other side, an opposed face 52 in contact with thestrain relaxation layer 6, in such a way that this thin film 5 has astrong tendency to relax, this having the effect of straining therelaxation layer 6. What is obtained is the intermediate heterostructure8.

As shown in FIG. 1H a thermal budget called “relaxation budget” is thenapplied to this heterostructure 8 so as to bring it to at least therelaxation temperature. As a reminder, it will be recalled that athermal budget corresponds to a heat treatment temperature for theperiod of application of this treatment. The thermal budget is appliedat least at the relaxation temperature and for the time needed to causethe plastic deformation of the relaxation layer 6 of crystallinematerial and the elastic deformation, for at least partial relaxation,of the thin film 5. This time may be shortened if the temperature of theapplied heat treatment is increased.

The term “partial relaxation” is understood to mean that the elasticdeformation of the material does not allow its nominal lattice parameterto be reached, to within the measurement uncertainties. The relaxationof the thin film 5 may also be complete. A layer of material is said tobe in the “relaxed state” if its lattice parameter corresponds to itsnominal lattice parameter, taking into account the measurementuncertainties and the measurement method used.

To obtain plastic deformation, the relaxation layer materials are heatedto the following temperatures: at least 700° C. in the case of silicon;600° C. in the case of germanium; 500° C. in the case of galliumarsenide; and 500° C. in the case of indium phosphide. The time forwhich these temperatures are applied can be easily determined by aperson skilled in the art, these varying from a few seconds to a fewhours respectively.

Following the application of this heat treatment, the strain relaxationlayer 6 deforms plastically as explained above, for example via thegeneration of defects or fissures. Conversely, the thin film 5 relaxesby elastic deformation, without the formation of dislocations, until astrain energy balance is achieved and dislocations are formed in thematerial of the strain relaxation layer 6, the balance being achievedwhen the strain energy of the thin film 5 is no longer sufficient toform new dislocations during the heat treatment.

A relaxation saturation phenomenon may also take place. Dislocations areformed and relax the strain up to a certain threshold, which is reachedwhen the dislocations come together in the crystal and are annihilated,improving the crystalline cohesion. Relaxation by dislocation growth istherefore at saturation.

The thin film 5 is considered to deform elastically as it could intheory return to its initial state, unlike the strain relaxation layer6, if the strains exerted by the film 5 disappear. Once the temperaturehas returned to the initial temperature after applying the thermalbudget, the material of the layer 6 can no longer return to its initialstate as it has undergone a plastic deformation, and thus the thin film5 remains in the relaxed or partially relaxed state. What is thusobtained is the final heterostructure 8′.

The process according to the invention has the advantage that the thinfilm 5 relaxes by the formation of dislocations in the relaxation layer6 and not in the film 5, unlike in the case when a thin film is formedepitaxially, directly on a layer that has a different lattice parameter.Thanks to the process according to the invention, the thin film 5 isrelaxed in the final state and it also has the desired crystallinequality, which quality is desired in particular for a high-qualityepitaxy to be carried out subsequently in order to deposit active layersof optoelectronic or photovoltaic components, or else to form electronicdevices.

According to a variant of the process, a step of forming trenches in thethickness of the film 5 may be carried out before the relaxation heattreatment. This step may be performed by etching and results in theformation of islands of materials in the thin film 5. The size of theseislands may vary for example from 100 μm×100 μm to several mm².

This technique prevents any pleating of the thin film 5 occurring duringthe relaxation. Pleating occurs in particular when the thin film 5 isinitially highly strained. Moreover, the formation of islands enablesthe thickness of the strain relaxation layer 6 to be reduced to thedeformation of each of the islands rather than to the total deformationof the thin film 5.

The process according to the invention is also advantageous in that adouble transfer of the thin film 5 is used. This is because, in theparticular case in which the thin film 5 consists of a polar III/Nmaterial, for example indium gallium nitride (InGaN) and when thematerial of this thin film 5 has an upper face with the polarity ofgallium or the polarity of the III element at the end of the firstepitaxy carried out in FIG. 1A, the double transfer of the subsequentfilm makes it possible to have the upper face 51 with the polarity ofgallium or of the III element on the top of the heterostructure 8 or 8′obtained, this polarity being more favourable to subsequent epitaxialregrowth.

If the material of the thin film 5 is not polar, it is unnecessary tocarry out the double transfer. It is then possible to deposit the thinfilm 5 directly on the intermediate substrate 2 by epitaxy. This epitaxyon the intermediate substrate 2 makes it possible to avoid having totransfer the epitaxially grown thin film 5 firstly to the substrate 1.In this case, the intermediate substrate 2 will be chosen so that itpreferably has a lattice parameter mismatch with the material of thethin film 5 of at least 0.13%. The strain relaxation layer 6 is thendeposited on the film 5, and then the film 5 and the layer 6 aretransferred onto the final substrate 3, before the relaxation treatmentis carried out.

The lattice parameter mismatch of 0.13% between the thin film 5 and theinitial epitaxy substrate 1, or between the thin film 5 and theintermediate substrate 2 when film 5 is deposited directly on it (andnot transferred), ensures sufficient elastic energy and force within thefilm 5 to cause it to relax after the relaxation thermal budget has beenapplied. It will be noted that in the case in which the film 5 hasfirstly been deposited epitaxially on the substrate 1, it is unnecessarywhen transferring it onto the intermediate substrate 2 for the latter tohave a lattice parameter mismatch of at least 0.13%.

As explained above, the final heterostructure 8′ obtained mayadvantageously be used as an epitaxy substrate. Two alternative ways ofcarrying out this epitaxy will now be described in conjunction withFIGS. 2A to 2D and 3A to 3C. The epitaxy may be formed with a materialdiffering in nature from the material of the thin film 5.

As may be seen in FIG. 2B, a layer of material 80, called an “epilayer,”can be epitaxially deposited on the thin film 5 of the finalheterostructure 8′. The thin film 5 then serves as a seed film.Preferably, the material of the epilayer 80 has a lattice parameterclose to that of the thin seed film 5, preferably with a latticeparameter difference of less than 0.5%. More preferably, the epilayer isa homoepitaxial layer.

It is sometimes necessary for the thin seed film 5 to have a lowdislocation density so as to obtain a layer of material 80 having goodcrystallinity, that is to say a low defect density. In this case, andprovided that the lattice parameters of the thin film 5 are close tothose of the epilayer 80, it is possible to obtain an epilayer 80 withan advantageously low dislocation density, i.e., in the case of GaN forexample, less than 10⁷ dislocations/cm². For a material such as InGaNwith an indium content of 10%, the dislocation density may be less than5×10⁸ dislocations/cm². Also advantageously, the thermal expansioncoefficient of the mechanical substrate 3 is chosen so as to be close tothat of the epilayer 80 so as to prevent fissures during cooling.

If the thermal budget applied during the epitaxy of the layer 80corresponds to that needed to cause plastic deformation of the strainmatching layer 6 and elastic deformation of the film 5, as mentionedabove, the epitaxy may be carried out on the intermediateheterostructure 8 shown in FIG. 2A. In this case, after the epitaxy, theheterostructure 8 becomes the final heterostructure 8′ as shown in FIG.2B.

The epilayer 80 may include at least one active layer. After theepitaxy, the layer 80 may be transferred onto a definitive substrate 4using a bonding layer 74 and then the substrate 3 can be removed, forexample, using a mechanical thinning technique such as polishing.Subsequent finishing steps enable the film 5 and layer 6 to be removed(see FIG. 2D).

Another implementation method, shown in FIGS. 3A to 3C, differs fromthat which has just been described in that the thickness of the assemblycomprising the film 5, the layer 6 and the epilayer 80 is sufficient tobe self-supporting. In this case, the mechanical substrate 3 is removed,for example by polishing or any other technique well known to thoseskilled in the art and appropriate for this removal.

One particular exemplary embodiment of the invention will now bedescribed. An indium gallium nitride (InGaN) film 5 containing 10%indium is deposited epitaxially on an epitaxy substrate 1, consisting ofsapphire covered with a gallium nitride (GaN) layer. This epitaxy iscontinued until the InGaN film 5 reaches a thickness of 50 nanometres.This film has a lattice parameter difference of about 1% from that ofthe gallium nitride on which it rests. The diameter of the film 5 is50.8 mm (or 2 inches). It has a dislocation density of less than5×10⁸/cm².

An SiN layer is then deposited on the InGaN film 5 and then covered withan SiO₂ bonding layer 71, before it is bonded to a substrate 2.

Next, hydrogen ions or hydrogen and helium ions are implanted with atotal dose of between 2.5 and 6×10¹⁷ atoms/cm² into the GaN through theInGaN film, to form therein a weakened zone 10.

Next, that surface of the InGaN film 5 which is covered with the SiN andSiO₂ layers is bonded to a sapphire intermediate substrate 2. A fracturethermal budget is applied to this structure and the 50 nanometre InGaNfilm 5 is transferred onto the intermediate substrate 2.

After removing the residual GaN from the film 5, a polycrystallinegermanium strain relaxation layer 6 is then deposited on the InGaN film5, which has a nitrogen polarity, until a thickness of at least 10microns is reached.

Next, a possible SiN adhesion layer is deposited thereon, followed by asilicon oxide bonding layer 72.

Moreover, a final sapphire substrate 3 is prepared, sapphire beingchosen as it has a thermal expansion coefficient matched to theenvisaged subsequent epitaxy. An SiO₂ bonding layer is also deposited onthis final substrate 3. The surfaces to be mated are planarized and thenbrought into contact so as to carry out the bonding.

Next, the intermediate sapphire substrate is removed by etching the SiO₂bonding layer, so as to detach this substrate 2. The upper face 51 ofthe InGaN film 5 of gallium polarity is cleaned of the residues of thebonding layer 71.

An etching step is then carried out over the thickness of the thin film5 in order to form InGaN islands measuring 100 μm×100 μm.

Next, an 800° C./4 hour thermal budget is applied to the heterostructure8 obtained, until the germanium relaxation layer 6 undergoes plasticdeformation and the thin InGaN film 5 undergoes complete relaxation.

An epilayer 80 of indium gallium nitride with a 10% indium content isformed on the thin seed film 5 so as to obtain a high-quality materialhaving a dislocation density of less than 5×10⁸/cm².

1. A process for fabricating a heterostructure comprising a multilayerstack in which an upper layer comprises an at least partially relaxedthin film of crystalline material, the process comprising: providing athin film of crystalline material in a strained state on an intermediatesubstrate; depositing a strain relaxation layer comprising a layer ofcrystalline material on the thin film of crystalline material; selectingthe crystalline material of the strain relaxation layer to beplastically deformable by a heat treatment at a relaxation temperatureat which the crystalline material of the thin film of crystallinematerial deforms by elastic deformation; transferring the thin film ofcrystalline material and the strain relaxation layer together onto asupport substrate to form an intermediate heterostructure; and heatingthe intermediate heterostructure to at least the relaxation temperatureand causing plastic deformation of the strain relaxation layer and atleast partial relaxation of the thin film of crystalline material byelastic deformation to provide the heterostructure.
 2. The process ofclaim 1, wherein heating the intermediate heterostructure comprisescompleting relaxation of the thin film by elastic deformation.
 3. Theprocess of claim 1, further comprising selecting a thickness of thestrain relaxation layer to be at least equal to a length of adeformation of the thin film of crystalline material when the thin filmof crystalline material relaxes by elastic deformation.
 4. The processaccording claim 1, further comprising etching the thin film ofcrystalline material to form islands of material therein prior toheating the intermediate heterostructure.
 5. The process of claim 1,wherein providing the thin film on of crystalline material in thestrained state on the intermediate substrate comprises epitaxiallydepositing the thin film of crystalline material on an initial epitaxysubstrate in the strained state, and subsequently transferring the thinfilm of crystalline material from the epitaxy substrate to theintermediate substrate.
 6. The process of claim 5, wherein transferringthe thin film of crystalline material from the epitaxy substrate to theintermediate substrate comprises implanting ionic species into the thinfilm and forming a weakened zone within the initial epitaxy substrate,bonding the strained thin film of crystalline material to theintermediate substrate, and heating at least one of the thin film ofcrystalline material and the initial epitaxy substrate to detach thethin film of crystalline material from at least a portion of the initialepitaxy substrate.
 7. The process of claim 1, further comprisingdepositing the thin film on the intermediate substrate and forming thethin film of crystalline material to have a lattice parameter thatdiffers by at least 0.13% from a lattice parameter of a material of theintermediate substrate.
 8. The process of claim 5, further comprisingforming the thin film of crystalline material to have a latticeparameter that differs by at least 0.13% from a lattice parameter of amaterial of the initial epitaxy substrate.
 9. The process of claim 1,wherein providing the thin film of crystalline material in the strainedstate on the intermediate substrate comprises one of transferring anddepositing the thin film of crystalline material onto a removablesubstrate.
 10. The process of claim 1, wherein heating the intermediateheterostructure comprises epitaxially depositing an epilayer on the thinfilm of crystalline material.
 11. The process according to claim 10,further comprising selecting the epilayer to comprise an active layer.12. The process according to claim 1, further comprising forming thethin film of crystalline material to comprise a single crystal.
 13. Theprocess according to claim 1, wherein depositing the strain relaxationlayer comprises selecting the strain relaxation layer to comprise amaterial selected from the group consisting of silicon, germanium,indium phosphide and gallium arsenide.
 14. The process according toclaim 1, further comprising selecting the thin film of crystallinematerial to comprise a III/N type material.
 15. The process of claim 14,further comprising forming an upper face of the thin film of crystallinematerial to have he polarity of a III element of the III/N typematerial.
 16. A heterostructure for use in at least one of an electronicdevice, an optoelectronic device, an optic device, and a photovoltaicdevice, the heterostructure comprising a strain relaxation layercomprising a crystalline material, the strain relaxation layerinterposed between a mechanical support substrate and a thin film ofcrystalline material in a strained state, the crystalline material ofthe strain relaxation layer being plastically deformable by a heattreatment at a relaxation temperature at which the crystalline materialof the thin film elastically deforms, a thickness of the strainrelaxation layer being at least equal to a length of a deformation ofthe thin film that occurs when the thin film relaxes by elasticdeformation.
 17. (canceled)
 18. The heterostructure of claim 16, whereinthe thin film is in direct contact with the strain relaxation layer. 19.The heterostructure of claim 16, further comprising at least oneepilayer comprising a material in a relaxed state on the thin film ofcrystalline material, and wherein the crystalline material of the thinfilm comprises a single-crystal.
 20. The heterostructure of claim 16,wherein the crystalline material of the thin film comprises a III/N typematerial.
 21. The heterostructure of claim 20, wherein an upper face ofthe thin film has a polarity of a III element.
 22. The heterostructureof claim 16, wherein the crystalline material of the strain relaxationlayer comprises at least one of silicon, germanium, indium phosphide andgallium arsenide.
 23. The heterostructure of claim 16, wherein thecrystalline material of the thin film comprises InGaN and thecrystalline material of the strain relaxation layer comprises germanium.24. A heterostructure for use in at least one of an electronic device,an optoelectronic device, an optic device, and a photovoltaic device,the heterostructure comprising a strain relaxation layer comprising aplastically deformed crystalline material, the strain relaxation layerinterposed between a mechanical support substrate and a thin film of atleast partially relaxed crystalline material, the crystalline materialof the strain relaxation layer plastically deformed by a heat treatmentat a relaxation temperature at which the crystalline material of thethin film elastically deformed, a thickness of the strain relaxationlayer being at least equal to a length of a deformation of the thin filmthat occurred when the thin film relaxed by elastic deformation.